A switch device that switches data packets flowing through a communication network sometimes becomes congested when data packets concentrate. In a known technology for resolving congestion in a switch device, a communication device serving as a transmitter of data packets is caused to reduce the current congestion window size to a predetermined size when congestion occurs. It is also known that the flows in the Internet traffic have a data packet distribution in which a large number of mouse flows, each containing a small number of data packets, are intermixed with a small number of elephant flows, each containing a large number of data packets. Related art examples are disclosed in Japanese Laid-open Patent Publication No. 2013-191931 and Japanese Laid-open Patent Publication No. 2009-177658.
When the size of the congestion window is reduced when congestion occurs, data packets will be transmitted at a lower transmission rate. If the data packets are transmitted at a lower transmission rate, it will take a longer time for the data transmission to be completed for an elephant flow with a larger amount of data, compared with a mouse flow with a smaller amount of data. The throughput of the data transmitted by an elephant flow is therefore affected more, when the data packets included in a mouse flow and those included in an elephant flow are controlled consistently by reducing the congestion window size. Furthermore, when a smaller congestion window size is used, even a mouse flow with a small amount of data is transmitted at a lower transmission rate, and the resultant throughput might be reduced. In this manner, congestion control using a congestion window has been sometimes difficult to handle data packets efficiently.